Geochronology and geochemistry of the granites from the Jiabusi Ta-Nb-(Li-Rb-Cs) deposit at the northern margin of the North China Craton

Abstract

The newly discovered Jiabusi Ta-Nb deposit occurs within the latest Jurassic-Early Cretaceous multiphase granites, which are composed of felsic, peraluminous intrusions including biotite, two-mica, Li-mica, and topaz-lepidolite granites (abbreviated as BG, TMG, LAG, and TLG, respectively). Ore mineralizations are mainly hosted in the LAG and TLG, with subordinate mineralization occurring in the pegmatitic dykes. In this contribution, we present a systematic work of geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotope composition for determining the timing of mineralization, petrogenesis, geodynamic setting, and ore-forming processes. Zircon U-Pb dating constrains the emplacement ages of 148–151 Ma for the BG, TMG, and LAG and reveals at least three episodes of Ta-bearing granitic pegmatites formed at ∼150 Ma, 127 Ma, and ∼100 Ma, respectively. Disseminated cassiterite grains from the LAG, TLG, and KPD yielded U-Pb ages of 149, 149, and 104 Ma, respectively. These granites have similar and limited features of (87Sr/86Sr)i = 0.7099, εNd(t) = −2.8 to −5.1, and εHf(t) = −4.7 to +5.5 and relative old Nd and Hf model ages of 1166–1442 Ma and 840–1500 Ma, respectively, indicating that Jiabusi multiphase granites are likely derived from the same magma reservoir by the partial melting of Mesoproterozoic crust in the destroyed northern margin of the NCC and are characterized by having involved significantly juvenile crustal materials.From the BG, TMG to LAG and TLG, bulk-rock Li, Rb, Cs, Be, Sn, Nb, and Ta contents increase with decreased Zr/Hf, Nb/Ta, and K/Rb ratios. Such systematic variations denote that these multiphase granites are cogenetic and that the LAG and TLG have experienced a higher degree of magmatic fractionation than the BG and TMG. The decrease in bulk-rock (La/Yb) N ratios, ΣREE, and Sr-Ba concentrations from the BG to TMG and LAG suggest an evolution involving the fractional crystallization of monazite and feldspar. The remarkable M−type lanthanide tetrad effect (TE1,3 = 1.22–1.75) matches low Nb/Ta (<1.2) and Zr/Hf (<5.0) ratios, indicating both the magmatic fractionation and melt-hydrothermal interaction were closely associated with Ta-Nb super enrichment in the LAG and TLG stocks. The combined evidence of compositional gaps and geologic spatial relationship between the BG batholith and Ta-bearing plutons confirm that the western part of the Jiabusi LAG stock and its surrounding concealed muscovite granites (MSG) are the further prospecting targets for Ta-Nb and W deposits.